A method of in situ remediation of soil contaminated with chlorinated hydrocarbon solvents involves injection of nanometer-size iron particles. The present method exploits a combination of prompt chemical remediation followed by longer-term enhanced bioremediation and, optionally, is practiced in conjunction with the method of bioremediation described earlier. Newly injected iron particles chemically reduce chlorinated hydrocarbons upon contact. Thereafter, in the presence of groundwater, the particles slowly corrode via chemical reactions that effect sustained release of dissolved hydrogen. The hydrogen serves as an electron donor, increasing the metabolic activity of the anaerobic bacteria and thereby sustaining bioremediation at a rate higher than the natural rate.